This invention relates generally to gas turbine engines, and, more specifically, to lean premixed combustors used with gas turbines.
Many known combustion turbine engines ignite a fuel-air mixture in a combustor and generate a combustion gas stream that is channeled to a turbine via a hot gas path. The turbine converts the thermal energy of the combustion gas stream to mechanical energy that rotates a turbine shaft. The output of the turbine may be used to power a machine, such as an electric generator or a pump.
Environmental concerns regarding exhaust emissions generated from combustive processes have resulted in regulations and other limits on gas turbine engines. In response, at least some industrial gas turbine engines include a combustor designed for low exhaust emissions operation, for example, a lean-premixed combustor. Known lean-premixed combustors typically include a plurality of burner cans, or combustors, that circumferentially adjoin each other around the circumference of the engine, such that each burner can includes a plurality of premixers joined together at its upstream end.
However, lean premixed combustors may be more susceptible to combustion instability due to pressure oscillations in the combustion chamber. Such instabilities can cause undesirable acoustic noise, deteriorate engine performance and reliability, and/or increase the frequency of required service. For example, combustion instability can cause flashback, flame blowout, starting problems, damage to combustor hardware, switchover problems, High Cycle Fatigue (HCF) of hot gas path components, and Foreign Object Damage (FOD) to turbine components. If there is extensive structural damage, system failure can occur.
One known method for reducing combustion instabilities involves distributing the axial position of the flame in the combustion chamber by physically offsetting one or more fuel injectors within the combustion chamber. However, in such a combustor, the extended surface associated with the downstream injectors must be actively cooled in order to be protected from the upstream flame. This additional cooling air has corresponding NOx emissions for the system. Another known method involves changing the distance between the centerbody and the cap for different premixers. By altering such distances, the spatial distribution of heat release rates for each premixer can mitigate the feedback gain. However, this method can be time-consuming because each premixer, or nozzle assembly, has a different configuration and different orientations and may not work for all operating conditions.